GB2084774A - Vehicle axle sensor - Google Patents

Abstract

An axle sensor for use in a vehicle lane including a screened cable 4 encased in a sleeve 3 of force-transmitting medium. A groove 2 is cut across the vehicle lane 1 and the sleeve inserted into it so as to leave a portion standing proud of the surrounding lane surface. Pressure due to overpassing vehicles is transmitted through the sleeve and causes the cable to flex. The cable is constructed so that the screen lies between low friction cable coatings and so moves relative to them to induce triboelectric signals thereon when flexed. The sleeve may constitute a polyurethane block in which the screened cable is embedded and is preferably of 'T'-shaped cross section for insertion into a groove of rectangular cross section. Epoxy glue 9 forms a firm seal. The cable is connected to signal conditioning circuitry on a roadside verge via an impedance converter which may be inserted into the groove and a conducting line which may be laid under the surface of any vehicle lanes lying between the lane under test and the roadside verge. <IMAGE>

Description

SPECIFICATION Improvements in or relating to axle sensors The invention relates to sensors for sensing axles of vehicles moving over a surface.

Axle sensors have been used on roads in the past for measuring, for example, numbers of axles passing a position on the road, vehicle speed and axle weight. In order to perform these measurements a variety of axle sensors have been used which rely upon vehicle wheels running over detectors lying along the surface of the road.

A first such prior sensor which operates pneumatically employs a flexible, air-filled detector tube lying across the width of the traffic lane, the tube being plugged at one end and connected to pressure sensing apparatus housed in a road side cabinet at the other.

Vehicle wheels passing over the detector cause air pressure fluctuations inside which are recorded by the sensing apparatus. This sensor, however has a number of limitations, notably the fact that clips which secure the tube to the road surface are readily displaced in use and that the tube becomes stretched after a relatively small number of axles have passed over it. Additionally the detector has a tendency to bounce, resulting in confused signal reception in the sensing apparatus which may therefore not be able to distinguish between two separate axles passing the sensor close together. Further disadvantages are that it is not impervious to water and that the tube can be broken relatively easily, for example, as a result of heavy vehicles braking and locking the wheels whilst they are passing over the detector.This type of sensor therefore has limited useful life which may be of the order of one week on a busy road. Additionally, this first prior art sensor is not readily adaptable for use in a non-outside lane of a multi-lane carriageway. Considering, for example, correlation of signals transferred to sensing apparatus at the road side in a sensor for the centre lane of a three-lane carriageway; a means of adapting the basic sensor employs a first pneumatic tube lying across both the centre and an outer lane of the carriageway and transferring signals to sensing apparatus adjacent to the road and used in conjunction with a second pneumatic tube lying across the same outer lane and transferring signals to the same sensing apparatus. The signals arising from the latter tube are then subtracted from those arising from the forner tube to obtain a measure of the axle count in the centre lane.

Some of the disadvantages of this adaptation are that it is more likely than the basic sensor to fail as there is more tube on the road which is therefore more susceptible to breakage. Secondly, an accurate count is not obtainable if, for example, two cars pass the sensor simultaneously, one in the centre and one in the outer lane. Additionally, a relatively large quantity of tube is required for this adaptation.

A second sensor which has been used in the past consists of an elongated polyurethane block for insertion into a channel cut across the width of traffic lane and encasing an aluminium tube connected to an "anvil" support terminating in an aluminium surface which is designed to lie flush with the surface of the road. A loose cable rests in the aluminium tube such that, when a vehicle's wheels pass over the aluminium surface, noise signals are created on the cable as a result of its vibration inside the tube.

A number of disadvantages arise from this second sensor. A first such disadvantage is that the noise signal produced exhibits marked ringing. Two axles passing relatively closely spaced therefore may produce signals which are difficult to separate from each other. A second disadvantage arises as a result of the cable being free to move within the aluminium tube and deforming from a straight line to. a 'snaked' position under the influence of the vibrations. As it therefore has a non-uniform configuration across the traffic lane, the signal obtained from the cable varies according to the particular part of the lane which a vehicle passes. A third disadvantage is that the aluminium surface lying at road surface level has a flexibility which is insufficiently high to allow the aluminium to conform adequately to the lane's immediate shape.

A third and simpler prior art sensor employs a coaxial cable lying across the traffic lane and clamped to the surface by, for example, adhesive tape or securing-clips placed at intervals along its length. A vehicle axle passing over the cable creates a noise signal on the conducting wires which may be recorded on a recorder connected to the cable and housed at the side of the road.

Some of the disadvantages of this third prior art sensor are that the securing means becomes displaced relatively easily and also that the cable is vulnerable to breakage. Additionally, it is not readily adaptable for use in a traffic lane which is not an outer lane bordered by a roadside verge. To detect traffic in, for example, the centre lane of a three-lane carriageway, the signals may be passed to the roadside recorder by arranging a continuous length of coaxial cable to lie on top of the road surface across the centre lane then under the road surface across an adjoining outer lane and thence to the recorder. However, in order to avoid spurious signals arising from the outer lane, the cable must be deeply and firmly embedded under the surface, thus demanding an installation which is not costeffective having regard to the limited life of the centre-lane section.An alternative method of centre-lane adaptation employs a first coaxial cable lying above the surface of both the centre and outer lane in conjunction with a second coaxial cable lying above the surface of the same outer lane. As with the comparable adaptation of the prior art pneumatic sensor, a measure of axle count is obtained by subtracting the second cable's signals from those of the first. However, the same disadvantages apply to this adaptation of the basic third prior art sensor as to the comparable adaptation of the basic first prior art sensor.

Some of the objectives of the present invention are to provide an axle sensor which is hard-wearing, relatively iexpensive, readily adaptable for use in any lane of a multi-lane carriageway and capable of detecting closelyspaced axles.

According to the present invention there is provided an axle sensor for use in a vehicle lane including a screened cable encased in a sleeve of a force-transmitting medium for insertion into a groove across the lane such that triboelectric signals are induced on the cable by transmission of vibrations through the sleeve from overpassing vehicles.

The sleeve may constitute a polyurethane block and, advantageously, a "Flexane" (Registered Trade Mark) material manufactured by Devcon. Preferably the Flexane is of grade 80 for heavy vehicles and fast traffic and may be of grade 50 or 60 for detection of lighter and slower moving vehicles, for example bicycles, where no heavy vehicles or braking vehicles are overpassing.

The sleeve may be shaped to have a rectangular cross section and dimensioned so that, when inserted into the lane's groove, a small fraction of its height stands proud of the surrounding road surface. Preferably, however, the sleeve has a 'T'-shaped cross section such that the horizontal arms of the 'T' stand proud of and rest upon the surrounding road surface and the vertical section sits in the groove. The cable is preferably positioned within the sleeve so as to sit below the level of the surrounding road surface and may be moved nearer to the level of the surface for increased sensitivity.

The sleeve may be sealed into the groove using an adhesive material and preferably an epoxy glue when using a Flexane sleeve.

The cable may be connected to an impedance converter which is fitted into a casing sealed into the groove of the road at the end of the sleeved section of cable and which is connected by a conductor to signal conditioning circuitry at the roadside. For an axle sensor used in a traffic lane not bordered by a roadside verge the conductor may be laid under the surface of the adjacent lane(s) in order to reach the kerb and thence to the signal conditioning circuitry, the conductor being chosen so as not to produce spurious signals from traffic in any lanes which it traverses.

The invention will now be described by way of example only with reference to the accompanying diagrams of which Figure 1 illustrates a cross section through a first traffic lane bordered on one side by a second traffic lane and on the other side by a roadside verge at a position occupied by a sensor according to one aspect of the invention.

Figure 2 illustrates a cross section through a traffic lane of the same type as the first traffic lane of Fig. 1 at a position occupied by a sensor according to a second aspect of the invention.

Figure 3 is a block diagram illustrating electrical circuitry associated with the components of either Fig. 1 or Fig. 2.

Figure 4 illustrates the processing of a signal by the circuitry of Fig. 3.

In Fig. 1 a first traffic lane, 1, contains a groove, 2, cutting perpendicularly across the whole of the width of the lane between the roadside verge and the second lane and filled by a polyurethane block, 3, which forms a sleeve around a screened cable, 4. The block 3, which is composed of Devcon's "Flexane" of ASTM D41 2 hardness grade 80 is of rectangular cross section, the dimensions 5, 6, 7 and 8 associated with it being 1 2 mm, 12 mm, 32 mm and 3 mm respectively. The material, Flexane, is a hardwearing polyurethane which flexes if pressure is applied to it and which is not readily eroded by material, for example, petrol, oil or water, which may be present on the road.

The block, 3, is inserted into the road by first cutting away the groove, 2, with a cutting tool and pouring a liquid epoxy glue into the groove bottom. The block is then lowered into the groove before the adhesive sets, beginning from the end adjacent to the second lane and working gradually towards the end adjacent to the roadside verge so as not to trap air bubbles at any points beneath the block which would allow the urethane to flex excessively and may result in extra wear at such points. The epoxy glue then sets to form a firm seal, 9, around the block, 3, and to hold it in place in the groove.

At the end of the block adjacent to the second lane the screened cable, 4, is sealed by a cap which does not allow, for example, water to enter the cable.

The screened cable, 4, is type M1 4714/78 manufactured by Sterling Cable Company Ltd. and comprises a central copper core enveloped by a laminated insulation comprising "Kapton" (Registered Trade Mark of Du Pont) layers bonded together by "Tefcel" (Registered Trade mark of Du Pont) fluorinated ethylene propylene. This is surrounded by an overlapped, four-wire-weave tinned copper screen having a 0.9 filling factor which is surrounded by a film of "Mylar" (Registered trade mark of Du Pont) tape, this then being covered by an ethylene propylene rubber (EPR) protecting and insulating covering.

In operation a vehicle's wheels passing over the block, 3, in the traffic lane collide with the portion standing proud of the surrounding surface, and cause it to flex. This shock is transmitted through the urethane sleeve to the cable, 4, causing the cable to flex and the screen to move relative to the enveloping insulations. The relative movement is promoted by the extremely smooth surface of the Kapton/Tefcel insulation and the smooth film of Mylar and results in a triboelectric charge being set up on the cable screening. The smoothness of the insulations allows the cable to be flexed many times without any internal wear takng place.

Fig. 2 illustrates an alternative configuration for the block, 3. In this configuration the block has a 'T'-shaped cross section, the dimensions 10, 11, 12 and 1 3 being 5 mm, 3mm, 32mm and 1 2mm respectively. By virtue of the supporting arms of the 'T' this shape of block is easier to insert into the groove, 2, than the rectangular block. Additionally, the arms of the 'T' provide extra protection against seepage of water into the groove as a result of flexing, this water then being able to form ice at low temperatures and cause wear on the walls of the groove which may result in the block eventually breaking free.

The end of the screened cable, 4, adjacent to the roadside verge is connected to the circuitry of Fig. 3. The cable is first connected to an impedance converter, 14, fitted into a metal casing which is sealed into the groove, 2, at the end of the block, 3, adjacent to the roadside verge. The impedance converter, 14, is then connected by a conductor, 15, to signal-conditioning circuitry, 16, at the roadside. By including the impedance converter, 14, directly adjacent to the end of the screened cable, 4, the components of the axle sensor mounted in the traffic lane may be readily adopted for use with any length of conductor and form of signal-conditioning circuitry, 16.

The circuitry, 16, includes a low gain amplifier, 17, connected to an AC/DC converter, 18, which is connected to a DC amplifier, 1 9.

The output of the amplifier, 19, connects to a Schmitt trigger, 20, which then connects to a retriggerable monostable, 21, from which an output, 22, is taken and may be applied to, for example, a recorder. When the screened cable, 4, is connected to the circuitry of Fig.

3, a triboelectric charge induced on the cable by overpassing vehicle tyres forms a signal at the input to the low gain amplifier, 17, as in Fig. 4(a) (which illustrates the signals arising from two overpassing axles). This results in signals as in Fig. 4(b) at the input to the converter, 1 8. The signals are then rectified by the converter, 18, as in Fig. 4(c) and passed to the Schmitt trigger which has a trigger level set according to prevailing conditions so as to produce square wave signals as in Fig. 4(d). These then enter the retriggerable monostable which has a retriggering delay set according to prevailing conditions sufficiently long to not produce more than a single output signal for each axle overpass, but sufficiently short to not produce a single output for two overpasses spaced an average close distance together.Output signals as in Fig. 4(e) showing a sharp cut-off and having a duration related to that of vehicle tyre contact with the sensor is thus produced at output 22.

To adapt the axle sensor for use in a traffic lane not bordered by a roadside verge, the polyurethane block, 3, may be laid in a groove, 2, out across the required lane and the conductor, 15, laid underneath the road surface of adjacent lanes so as to reach the conditioning circuitry, 1 6, at the roadside, but so as to be unaffected by traffic in adjacent lanes. A groove across adjacent lanes to accommodate the conductor may be cut at the same time as groove, 2.

The sensor may be adapted for use in sensing slow traffic and traffic of low axle weight, for example, bicycles, by using a softer urethane. A grade 50 or 60 Flexane may, for instance, be used. However, as the grade of the Flexane decreases, its durability decreases. Thus, grades 50 and 60 may have limitingly short lives, particularly if heavy traffic in addition to light traffic is overpassing and especially if the traffic is applying brakes over the sensor.

The invention is not confined to the details of the above embodiments, variations being apparent to those skilled in the art. The composition and the dimensions of the block, 3, may, for example, be varied. A shape other than rectangular or 'T'-shaped may be used and the depth of the coaxial cable below road surface level may be decreased to increase sensitivity or increased to decrease sensitivity.

The parameters of the coaxial cable may be varied from those described above. A very smooth insulation other than Kapton/Tefcel may be used, a screen with a higher filling factor may be employed to decrease its flexing power, or a screen with a lower filling factor may be used for increased flexing power for example. The impedance converter may be positioned in the groove at the end of the block adjacent to the second lane instead of at the roadside end. Alternatively, for easier servicing, the impedance converter may be positioned on the verge at the side of the road instead of in a groove in the road.

In the Fig. 1 embodiment the cable is laid across the whole of the width of the traffic lane. However, it may be optionally laid across only a part of the width of the lane which traffic overpasses.

Claims (11)

1. An axle sensor for use in a vehicle lane including a screened cable encased in a sleeve of force-transmitting medium for insertion into a groove across the lane such that triboelectric signals are induced on the cable by transmission of pressure through the sleeve from overpassing vehicles.

2. An axle sensor as claimed in claim 1 wherein the sleeve of force-transmitting medium comprises a urethane block in which the screened cable is embedded.

3. An axle sensor as claimed in either of the previous claims wherein the screen of the cable is wrapped between two insulating layers of cable at least one of which has a coefficient of friction sufficiently low to allow relative movement between the screen and the at least one of the two layers.

4. An axle sensor as claimed in any previous claim wherein the dimensions of the sleeve of force-transmitting medium are such that a portion of the sleeve stands proud of the vehicle lane surface when the sensor is inserted into the groove.

5. An axle sensor as claimed in claim 4 wherein the sleeve has a rectangular cross section for insertion into a groove having a rectangular cross section.

6. An axle sensor as claimed in claim 4 wherein the sleeve has a 'T'-shaped cross section for insertion into a groove having a rectangular cross section such that the horizontal arms of the T rest on the lips of the groove and stand proud of the vehicle lane surface.

7. An axle sensor as claimed in any previous claim wherein the screened cable is positioned within the sleeve so as to lie below the level of the vehicle lane surface when the sensor is inserted in the groove.

8. An axle sensor as claimed in any previous claims additionally including an impedance converter connected to one end of the screened cable for insertion completely inside the groove across the vehicle lane at an end of the sleeve of force-transmitting medium.

9. An axle sensor as claimed in claim 8 additionally including signal conditioning circuits for conditioning and recording triboelectric signals generated on the axle sensor and for housing on a roadside verge and conducting line connected between the conditioning circuits and the impedance converter.

10. An axle sensor as claimed in claim 9 for use in a first vehicle lane not bordered by a roadside verge wherein the screened cable is sufficiently long to straddle the first vehicle lane and the conducting line is sufficiently long to straddle other vehicle lanes lying between the first lane and a roadside verge.

11. An axle sensor substantially as herein described with reference to the accompanying drawings.

1 2. A method of inserting into a groove across a vehicle lane an axle sensor according to any previous claim substantially as herein described with reference to the accompanying drawings.